Every pokemon has special versions or "types" that can be caught but are a bit more rare than normal pokemon. Each of these versions fill a different spot in the pokedex, every version of each pokemon must be caught in order to complete the pokedex.
Here is the normal version:
Normal Pokemon have no changes.
This is the dark version:
Dark Pokemon deal 25% more damage in their attacks than Normal Pokemon.
This is the shiny version:
Shiny Pokemon have 25% more HP than Normal Pokemon.